There are people who are inexplicably scared of changing their endings to suit their beginnings. These are usually the same types of people who would rather throw their entire story out than deviate from the outline.
Imagine in chapter twelve your character meets an expert swordsman. Unfortunately, though your character must fight him for the plot, you never gave him a sword or the training to use it earlier! What do you do?
A lot of authors would either have him find a fortuitous sword on the floor or find a clunky way to fight or run that doesn’t involve swords at all. After all, nobody wants to have “All of the sudden I had a sword” in their story.
The easiest way to resolve this is the one people often don’t think of: let him fight with a sword and then edit the beginning so that he gets both the item and the prowess beforehand. It’s not a retcon if you do it before your story gets released to the general public!